v3.25.2
EQUITY - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 6 Months Ended 12 Months Ended
Jan. 28, 2025
Jun. 30, 2025
Jun. 30, 2024
Jun. 30, 2025
Jun. 30, 2024
Dec. 31, 2024
Class of Stock [Line Items]            
Common stock, shares outstanding   16,291,395 16,173,132 16,291,395 16,173,132 16,192,345
Common stock, shares issued   32,838 5,245 38,311 11,048  
Proceeds from Issuance of Common Stock       $ 2,000 $ 600  
Share based compensation expense         1,200  
Preferred Stock   $ 200 $ 200 200 200 $ 200
Share Sold   1,700 300 2,000 600  
Maximum            
Class of Stock [Line Items]            
Dividend declared   100 $ 100 100 $ 100  
Unitil Corporation            
Class of Stock [Line Items]            
Common stock offering price   $ 50,000   $ 50,000    
Restricted Stock            
Class of Stock [Line Items]            
Restricted stock vesting period       4 years    
Restricted stock non-vested   25,170 59,163 25,170 59,163  
Restricted Stock Units Granted       $ 50.85 $ 47.72  
Share based compensation expense       $ 1,300    
Fair value of liabilities associated with fully vested RSUs that will be settled in cash   $ 600 $ 800 600 $ 800 $ 900
Unrecognized share based compensation   $ 1,400   $ 1,400    
Share compensation recognition period       2 years 8 months 12 days    
Restricted Stock Units Granted 26,430          
Aggregate Market Value $ 1,400          
Forfeitures under the stock plan       396    
Cancellations under the stock plan       0    
Restricted Stock | Maximum            
Class of Stock [Line Items]            
Restricted stock available for awards   1,027,500   1,027,500    
Restricted stock that may be awarded in any one calendar year to any one participant   20,000   20,000    
Restricted Stock | Vesting Annually            
Class of Stock [Line Items]            
Restricted stock vesting percentage annually       25.00%    
Restricted Stock Units (RSUs)            
Class of Stock [Line Items]            
Percentage of fully-vested restricted stock units that directors will receive in common shares when settled       70.00%    
Restricted Stock Units Granted       0    
Restricted stock units outstanding     33,921   33,921  
Weighted-Average Stock Price     $ 42.89   $ 42.89  
Percentage of fully-vested restricted stock units that directors will receive in cash when settled       30.00%    
Performance Shares [Member]            
Class of Stock [Line Items]            
Restricted stock non-vested   67,380 43,453 67,380 43,453  
Restricted Stock Units Granted       $ 51.21 $ 50.35  
Share based compensation expense       $ 800 $ 500  
Unrecognized share based compensation   $ 2,500   $ 2,500    
Forfeited       250    
Share compensation recognition period       1 year 10 months 24 days    
Restricted Stock Units Granted 26,430          
Aggregate Market Value $ 1,400          
Cancellations under the stock plan       0    
Series 6 | Unitil Energy Systems Inc            
Class of Stock [Line Items]            
Preferred stock, outstanding   1,727 1,727 1,727 1,727 1,727
Preferred Stock   $ 200 $ 200 $ 200 $ 200 $ 200
Dividend rate       6.00% 6.00% 6.00%
At-the-Market Equity Offering Program            
Class of Stock [Line Items]            
Common Stock Available For Future Sales       $ 48,500    
At-the-Market Equity Offering Program | Common Stock            
Class of Stock [Line Items]            
Share sold under ATM program   27,620   27,620    
Sale of stock, Average price per share   $ 53   $ 53    
Net Proceeds   $ 1,400   $ 1,400    
Gross proceeds   $ 1,500   1,500    
Dividend and Distribution Reinvestment and Share Purchase Plan            
Class of Stock [Line Items]            
Proceeds from Issuance of Common Stock       $ 590,600    
Dividend and Distribution Reinvestment and Share Purchase Plan | Average            
Class of Stock [Line Items]            
Common stock price per share   $ 55.25   $ 55.25    
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock            
Class of Stock [Line Items]            
Common stock, shares issued       10,691